Digital picture-in-picture decoder

ABSTRACT

Methods and apparatus for implementing a reduced cost HDTV/SDTV video decoder are disclosed. The described joint video decoder is capable of decoding HDTV pictures at approximately the resolution of standard definition television pictures and can be used to decode HDTV and/or SDTV pictures. The described video decoder may be used as part of a picture-in-picture decoder circuit for providing picture-in-picture capability without providing multiple full resolution video decoders. The reduction in decoder circuit complexity is achieved through the use of a plurality of data reduction techniques including the use of a preparser, downsampling, and truncating pixel values.

RELATED APPLICATIONS

This patent application is a continuation of pending application Ser.No. 08/339,436, filed on Nov. 14, 1994, which is a continuation-in-partof pending U.S. patent application Ser. No. 08/320,481 filed on Oct. 11,1994.

FIELD OF THE INVENTION

The present invention is directed to video decoders and, moreparticularly, to methods and apparatus for implementing video decodersthat are capable of decoding high definition television ("HDTV") signalsand/or standard definition television ("SDTV") signals.

BACKGROUND OF THE INVENTION

The use of digital, as opposed to analog signals, for televisionbroadcasts and the transmission of other types of video and audiosignals has been proposed as a way of allowing improved picture qualityand more efficient use of spectral bandwidth over that currentlypossible using analog NTSC television signals.

The International Standards Organization has set a standard for videodata compression for generating a compressed digital data stream that isexpected to be used for digital television. This standard is referred toas the ISO MPEG (International Standards Organization --Moving PictureExperts Group) ("MPEG") standard. In accordance with the MPEG standard,video data is encoded using discrete cosine transform encoding and isarranged into variable length encoded data packets for transmission.

One version of the MPEG standard, MPEG-2, is described in theInternational Standards Organization--Moving Picture Experts Group,Drafts of Recommendation H.262, ISO/IEC 13818-1 and 13818-2 titled"Information Technology Generic Coding Of Moving Pictures and AssociatedAudio" (hereinafter "the November 1993 ISO-MPEG Committee draft") herebyexpressly incorporated by reference. Any references made in this patentapplication to MPEG video data is to be understood to refer to videodata that complies with MPEG-2 standards as defined in the November 1993ISO-MPEG Committee drafts.

MPEG video data may be used to support either high definition television("HDTV"), wherein the video frames are of higher resolution than thoseused in present NTSC signals, or what will be referred to as standarddefinition television ("SDTV"), e.g., television which has approximatelythe same resolution per frame as the existing analog NTSC standard.Because HDTV, which includes the proposed United States AdvancedTelevision Standard ("US ATV"), provides higher resolution than SDTV,more data is required to represent a HDTV frame than is required torepresent a SDTV frame. Accordingly, it is possible to transmit multipleSDTV signals in the same bandwidth required to support a single HDTVsignal.

MPEG Main Profile at Main Level ("MP@ML") specifies various requirementsfor an MPEG compliant standard definition television signal andassociated decoding equipment. MP@ML allows pictures as large as 720×576pels for a total of 414,720 pels per picture. The proposed standard forUS ATV allows for pictures as large as 1920×1080 pels for a total of2,073,600 pels per picture.

Because of the relatively large amount of data required to representeach frame of a HDTV picture, HDTV decoders must support much higherdata rates than SDTV decoders. The additional memory required by a HDTVdecoder, as compared to a standard SDTV decoder, and the increasedcomplexity associated with the inverse DCT circuit and other componentsof a HDTV decoder can make a HDTV decoder considerably more expensivethan an SDTV decoder.

In fact, the cost of memory alone may make a HDTV set incorporating aHDTV decoder prohibitively expensive for some consumers. It is expectedthat a fully MPEG compliant video decoder for HDTV will require aminimum of 10 MB of RAM for frame storage with a practical HDTV decoderprobably requiring about 16 MB of relatively expensive Synchronous DRAM.

Accordingly, there is a need for a method and apparatus that permits:(1) a simplification of the complexity of the circuitry required toimplement a HDTV decoder, (2) a reduction in the amount of memoryrequired to implement a HDTV decoder circuit, and (3) a single decoderthat is capable of decoding both SDTV and HDTV signals. Furthermore, itis desirable that the cost of such a decoder be low enough that it is ina range that would be acceptable to most consumers, e.g., approximatelythe cost of a SDTV decoder.

While various proposals call for transmitting only US ATV signals it hasalso been suggested that some digital SDTV signals be broadcast. Variouscombinations of broadcasting HDTV and SDTV signals are possible withmultiple SDTV shows being broadcast during a particular time of the dayand a single HDTV broadcast being transmitted in the same bandwidth usedfor the SDTV signals at a different time of the day.

Thus, in order to remain compatible with both HDTV and SDTV broadcasts,there would be value in a television receiver include a video decodercapable of decoding both HDTV and SDTV signals and furthermore, thatsuch a video decoder be capable of being implemented at a relatively lowcost.

In addition, there is a need for a method and apparatus for implementingpicture-in-picture capability in a digital television without incurringthe cost of multiple full resolution decoders. In known analogpicture-in-picture systems, during picture-in-picture operation, andfull resolution decoder is normally used to decode the TV signal used toproduce a main picture and a second full resolution decoder is used todecode the television signal which is used to provide the second picturedisplayed within a small area of the main picture.

SUMMARY OF THE PRESENT INVENTION

The present invention is directed to video decoders and, moreparticularly, to methods and apparatus for implementing video decodersthat are capable of decoding high definition television ("HDTV") signalsand/or standard definition compressed video signals.

In particular the present invention is directed to a plurality ofmethods and apparatus for reducing the complexity of digital videodecoder circuitry and for reducing video decoder memory requirements. Inaccordance with the present invention, in order to reduce the costassociated with implementing a joint HD/SD television decoder, thedecoder of the present invention is designed to optimize the amount ofcircuitry that is used during both HD and SD television decoderoperation.

A HDTV decoder capable of decoding HDTV signals at less than full HDTVresolution, e.g., at approximately SDTV resolution and/or decoding SDTVsignals is described. The decoder of the present invention can also beused to decode SDTV signals at full SD resolution. It is expected thatas a result of the various data reduction features and circuitsimplifications of the present invention a combined HDTV/SDTV decoderthat outputs pictures at approximately SDTV resolution can be built atapproximately 130% of the cost of a digital SDTV decoder.

In accordance with one embodiment of the present invention the describedreduced resolution HDTV video decoders are used as part of apicture-in-picture decoder circuit for providing picture-in-picturecapability without the use of multiple full resolution video decoders.

The decoder of the present invention achieves substantial reductions incircuit complexity and memory requirements through the use of aplurality of data reduction techniques including the use of a preparser,downsampling, and truncating bytes of data representing pixel values sothat the pixel values are represented using only 5, 6 or 7 bits asopposed to, e.g., 8 bits per DCT value.

In accordance with one embodiment of the present invention, thepreparser is used to discard data to permit a smaller channel bufferthan would otherwise be required to be used. In addition, the preparserdiscards run-length/amplitude symbols for each macroblock, e.g., therun-length/amplitude symbols for each luminance or chrominance block ofa macroblock, when the number of run-length/amplitude symbols exceeds amaximum preselected number per macroblock and/or block. In oneembodiment the preparser also limits the number of bits output per unittime period, e.g., by limiting the number of bits per macroblock, suchthat a predetermined data processing rate which is determined by thecombined capability of the coded data buffer and syntax parser andvariable length decoder circuit is not exceeded. In this manner, thepresent invention permits a simpler real-time syntax parser and variablelength decoder circuit to be used than would be possible without suchpreparsing.

In accordance with the present invention, an inverse quantizer circuitand an inverse discrete cosine transform circuit are designed to processonly a preselected set of DCT coefficients corresponding to eachmacroblock and to set the rest of the coefficients, e.g., the DCTcoefficients removed by the preparser, to zero. In this manner thecomputational requirements of the inverse quantizer circuit and inversediscrete cosine transform circuit are substantially reduced allowing forsimpler circuit implementation.

In addition to the use of the preparser, the use of a downsampler and/ora least significant bit truncation circuit provide for furtherreductions in the amount of data used to represent each video picturethereby resulting in a significant reduction in frame buffer memoryrequirements.

In order to compensate for the effect of data reduction, includingdownsampling and data elimination, the decoder of the present inventionmay include low pass filters.

The above described features and embodiments of the present inventionalong with numerous other features and embodiments are described below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a video decoder in accordance with one exemplaryembodiment of the present invention.

FIG. 2A illustrates a video decoder in accordance with a secondexemplary embodiment of the present invention.

FIG. 2B illustrates a video decoder circuit capable of decoding bothHDTV and SDTV video signals in accordance with the present invention.

FIGS. 3A through 3E illustrate 8×8 blocks of DCT coefficientscorresponding to a HDTV macroblock in accordance with differentembodiments of the present invention.

FIG. 4 illustrates an exemplary circuit for decoding video images tosupport picture-in-picture television operation in accordance with thepresent invention.

DETAILED DESCRIPTION

The present invention is directed to video decoders and, moreparticularly, to a low cost video decoder that is capable of decodinghigh definition and/or standard definition digital video signals. Inaccordance with the present invention decoded HD pictures will bedisplayed with less than full HD resolution, e.g., at approximately SDresolution, while SD pictures will be displayed at full SD resolution.The decoder of the present invention can be implemented as a low costHDTV decoder or as a joint HD/SD television decoder. While the detaileddescription of the present invention is directed primarily to exemplaryembodiments of HDTV, SDTV and combined HDTV/SDTV decoders, it is to beunderstood that the numerous features of the present invention may beused to provide low cost reduced resolution digital video decoderssuitable for decoding digital pictures in general, and that thisdisclosure is therefore not limited solely to SDTV and HDTV decoders.Furthermore, while the exemplary embodiments are described as MPEGcompatible decoders it should be noted that the decoders of the presentinvention can be implemented to decode a plurality of MPEG like videodata streams that use discrete cosine transform data encoding.

In the case of television sets with relatively small screens, e.g., lessthan 20 inch screens, the difference in perceived resolution and picturequality between a HDTV signal and an SDTV signal will be barelynoticeable. Accordingly, low cost HDTV decoders in accordance with thepresent invention which provide less than full HDTV picture resolutionand quality are particularly well suited for such applications becauseof their cost advantages. The decoder of the present invention is alsowell suited for providing picture-in-picture capability because of thesmall display area of the second picture being displayed and therelatively low cost of the decoder of the present invention as comparedto the cost of a full HDTV decoder.

The method of the present invention for decoding HD and SD picturesincludes the steps of reducing the resolution of received HD picturesprior to decoding by using, e.g., a preparser unit and/or adaptivefield/frame downsampling to reduce the complexity of later processingstages of the decoder. In addition, by using an inverse discrete cosinetransform ("IDCT") circuit designed to maximize the amount of commoncircuitry used for processing both reduced resolution HD and SD picturesthe cost of implementing a combined HD/SD decoder circuit is reduced

Referring now to FIG. 2, there is illustrated a video decoder generallyindicated by the reference numeral 100, implemented in accordance withone embodiment of the present invention. The illustrated decoder 100 iscapable of decoding HD and/or SD television signals, e.g., MPEGcompliant television signals. When used as a HD decoder, the decodercircuit 100 can produce images from a HDTV signal that are comparable inquality or slightly better than SD television images.

The SD/HD video decoder circuit 100 comprises a preparser 112, a syntaxparser and variable length decoder circuit 120, an inverse quantizer122, an inverse DCT circuit 124, a down sampler 126, a summer 128, aswitch 129, a post processor 136, a motion compensated prediction("MCP") circuit 130, and a memory circuit 114 including a coded databuffer and a frame buffer 118.

Methods of reducing the overall cost of a HDTV decoder will now bedescribed with reference to the embodiment of FIG. 1.

Generally, the single most expensive element of a video decoder, interms of cost, is the frame memory 118 which may comprise, e.g., 16 MBof synchronous DRAM in a HD decoder. The coded data buffer 116, which isused for the temporary storage of the compressed bitstream represents asmaller but not insignificant cost. A fully MPEG compliant HDTV decoderis expected to require at least 1 MB of RAM for use as a coded databuffer.

Other elements of a decoder which add significantly to the cost of thedecoder are the inverse discrete cosine transform circuit ("IDCT") 124and the inverse quantizer ("IQ") circuit 122. The IDCT circuit 124 of aHDTV decoder is required to perform a large number of arithmeticcomputations at a high rate and therefore is likely to represent asignificant portion of a decoder's circuitry. The IQ circuit 122performs a smaller number of computations than the IDCT circuit 124 butbecause of the high speed and complexity of the computations, the costof the IQ circuit 122 may also be a significant component of a HDTVdecoder's overall cost.

In addition to the IDCT circuit 124 and the IQ circuit 122 motioncompensation and interpolation logic may require a significant amount oflogic gates, e.g., the motion compensation and interpolation logic mayrequire more logic gates than the IQ circuit to implement but less thanthe IDCT circuit. The syntax parser and variable length decoder circuit120 may also require a significant amount of logic gates to implement.

Because the cost and complexity of a HDTV decoder is largely a functionof the requirement that it process large amounts of data on a real timebasis, it is possible to reduce the complexity and thus the cost of aHDTV compatible decoder by reducing the amount of data that needs to beprocessed. While using only a small portion of the video data receivedin an HDTV signal will result in reduced resolution and picture quality,by carefully selecting which HDTV data to process and the method bywhich it is processed, video image quality comparable to or better thanSD television signals can be achieved. As will be discussed below, thepreparser 112 serves to dynamically limit the amount of video datasupplied to the remaining elements of the decoder circuit 100 includingthe syntax parser and VLD circuit 120 thereby reducing the amount ofdata that must be processed by the subsequent circuit elements on a realtime basis and the required complexity of those circuit elements. Anadditional benefit of the use of the preparser 112 is that it permitsfor the use of a smaller coded data buffer 116 than would otherwise berequired.

As illustrated in FIG. 2, the preparser 112 has a first input forreceiving the variable length encoded bitstream representing, e.g., aHDTV signal output by a transport demultiplexer/decoder circuit of areceiver, and a second input for receiving data rate controlinformation. The output of the preparser 112 is coupled to the input ofthe coded data buffer 116 which has a rate control output coupled to thesecond input of the preparser 112.

The preparser 112 operates to reduce the amount of HDTV data supplied tothe coded data buffer. It does this by selectively discarding some ofthe received HDTV data. By using the preparser 112 to parse and discarddata prior to variable length decoding it is possible to operate thepreparser 112 at a data rate that is a function of the data rate of thereceived bitstream. This rate will be considerably lower than the datarate that the corresponding circuitry would have to be capable ofsupporting if it were located after the variable length decoder circuit120. By preparsing the data prior to storing in the coded data buffer116 and syntax parsing and variable length decoding, the need to supportthe worst case processing requirements that might otherwise beencountered by the SP and VLD circuit 120 is avoided. Since the maximumdata rate, e.g., the worst case data rate, that the SP and VLD circuit120 would normally have to be designed to support can be as much as 60times higher than the average data rate, a substantial reduction in thecomplexity of the SP and VLD circuit 120 can be achieved by using thepreparser 112 to insure that support for what would normally be theworst case variable length decoding scenario need not be provided.

The preparser 112 parses the incoming bitstream, without performing acomplete variable length decode operation, to identify datacorresponding to different types of video frames, such asbi-directionally coded ("B-") frames, predictively coded video frames("P-") frames and intra-coded ("I-") frames, and to identify MPEG codingelements such as macroblocks including motion vectors and DCTcoefficients associated with each of the identified frames in thereceived data stream.

The preparser 112 performs several data reduction operations on a HDTVbitstream as required to insure that the processing capacity of the SPand VLD circuit 120 and throughout capability of the coded data buffer116 are not exceeded. Thus, the preparser 112 operates to insure is thatthe preparser output data rate stays within a real time decodingenvelope established by the decoding limits of the other components ofthe decoder 100. By reducing the data rate as required, the preparser112 also serves to reduce the coded data buffer size requirementsbecause the coded data buffer 116 need only be capable of storing areduced amount of data.

In one embodiment, to insure that the preparser output data rate stayswithin the preselected limits of the decoding envelope, the preparser112 limits the number of DCT coefficients used to represent a macroblockand/or a block of a macroblock to a preselected number and also limitsthe number of bits per macroblock and/or block of a macroblock byremoving the highest order DCT coefficients associated with a receivedmacroblock and/or block until the desired number of bits per macroblockand/or block required to prevent the coded data buffer capacity anddecoding limits from being exceeded is achieved. Both the number of DCTcoefficients per macroblock and the number of bits per macroblock and/orblock can be preselected to approximately achieve the desired data rateor can be dynamically selected by the preparser 112. Thus, it ispossible to insure that the output data rate of the preparser stayswithin the decoding envelope by operating directly on macroblocks as awhole or on the individual blocks which comprise the macroblock whereeach macroblock includes one or more luminance and chrominance blocks.

Thus, in one embodiment, to limit its output data rate to a levelrequired to prevent the coded data buffer 116 from overflowing, thepreparser 112 limits the number of DCT coefficients which are used torepresent each macroblock and/or block of a macroblock by discarding thehigher frequency DCT coefficients which exceed a preselected number ofDCT coefficients. In such an embodiment, removal of AC DCT coefficientsmay be performed preferentially on B- frames, then P- frees and then I-frames, with a preference for retaining data in earlier P- frames asopposed to later P- frames as measured from the most recently receivedI- frame, as required to achieve the desired reduction in the preparseroutput data rate.

In DCT based video compression systems the DCT coefficients arenaturally ordered in two dimensions according to vertical and horizontalfrequency bins. In typical systems the coefficients are assigned aone-dimensional ordering to allow for sequential transmission. MPEGallows for two different orderings, generally called zig-zag scan andalternate scan.

It may be the case that the subset of DCT coefficients which arepreserved, i.e., the DC DCT coefficients and lower frequency AC DCTcoefficients not discarded by the preparser 112 may not be consistentwith any linear range of coefficients in either of the MPEG scanningtypes. In such a case, it is difficult to limit the DCT coefficients tothe desired set by discarding all of the coded coefficients of amacroblock or block after a certain point in the data stream.

In accordance with the present invention, two different approaches areused to insure that the preparser 112 discards the preselected DCTcoefficients. In accordance with a first embodiment, the preparser 112chooses the coefficient index in scan order after which to discardcoefficients. In this embodiment the retained set of DCT coefficientswill initially include all coefficients which do belong to the desiredset as well as some coefficients which do not belong to the desired set.The preparser 112 then sends the DCT coefficients which include thosethat belong to the desired set to the coded data buffer 116 to be passedto the SP and VLD circuit 120.

In a second embodiment, the preparser 112 recodes the incomingbitstream. This is done by removing run length/amplitude codes whichrefer to undesired coefficients, and by then recoding the later desiredcoefficients with suitably modified run lengths.

By limiting the number of DCT coefficients per macroblock and/or block,the preparser 112 effectively discards runlength/amplitude symbols whicheffectively cause zero DCT coefficient values thus allowing for asimpler IDCT circuit 124 to be used, in some embodiments, as opposed tothe IDCT circuit that would be required to fully decode a HDTVbitstream.

The discarding of run-length/amplitude symbols also allows for a simplersyntax parser and variable length decoder circuit 120 than would otherwise be required because the use of the preparser 112 insures that nomore than the preselected number of run-length/amplitude pairs permacroblock and/or block will be output by the preparser 112 for furtherprocessing.

Yet another benefit of the discarding of some DCT coefficients, i.e.,the effective setting of such coefficients to zero, is that thepreparser 112 effects lowpass filtering on a macroblock basis therebyreducing or eliminating the need for additional filtering before adownsampling operation, which will be described below, is performed.

In one particular embodiment, the preparser 112 also operates to detectand eliminate data from the received bitstream which will have no impacton ultimate picture quality. In one such embodiment, the preparser 112deletes from the data stream such MPEG data as bit slice stuffing andstart code zero stuffing to thereby reduce the data rate withoutimpacting picture quality. By eliminating such unnecessary data, thepreparser output data rate and the size of the coded data buffer 116 canbe reduced in this manner with no impact on picture quality.

In one embodiment of the present invention, the preparser 112dynamically adjusts the amount of data discarding performed as afunction of the amount of memory available in the coded data buffer 116for storing the received data. In order to facilitate such dynamic datadiscarding the coded data buffer 116 includes a data rate monitoringcircuit which generates a signal supplied to the second input of thepreparser 112 indicating how much data is stored at any given time inthe coded data buffer 116.

In such an embodiment, the preparser 112 maintains a model of coded databuffer 116 occupancy and works to ensure that the buffer 116 will neveroverflow. As buffer occupancy increases as indicated by the signalreceived from the buffer 116, the preparser 112 becomes more severe inthe amount of high frequency information that is discarded furtherreducing the number of DCT coefficients per macroblock or block from apreselected maximum number to a lower number.

The preparser 112 can look to buffer occupancy characteristics of thereceived bitstream, e.g., the manner in which the video data wasoriginally encoded for data transmission, and use this information as afactor in determining what information should be discarded to achievethe required reduction in the data rate.

As one feature of the dynamic control of its output data rate and thusbuffer control, the preparser 112 computes and stores the number of bitsused to code each row of macroblocks that comprise a picture as the datais received. While this number may vary dramatically from row to row andpicture to picture, since the decoder will take roughly the same amountof time to process each row of macroblocks regardless of the number ofbits used to represent the row of macroblocks, the preparser can predictthe rate of code buffer emptying. When this rate is expected to berelatively fast, e.g., because a large number of bits are used torepresent the preceding rows of macroblocks, the preparser 112 candiscard less data from the received data stream without concern of thecoded data buffer 116 overflowing. However, when relatively few bits areused to code the macroblocks representing the preceding rows ofmacroblocks, the coded data buffer 116 will empty on a comparativelyslow basis requiring the preparser 112 to discard more data from thereceived data stream until the coded data buffer 116 becomes lessoccupied.

In one embodiment, to minimize implementation costs, the preparser 112,which may also be referred to as a bitstream preprocessor, is designedto run at the incoming bit rate and is implemented using a bit serialarchitecture. Alternatively, the preparser 112 can parse an entire tokenat once, and at a rate that never needs to exceed the data rate of thereceived variable length encoded bitstream divided by the minimum lengthof a symbol in the bitstream.

As will be discussed below, when the decoder circuit 100 is used as ajoint reduced cost HDTV decoder and SDTV decoder, the data throughput isselected to permit the passing of a full rate SDTV bitstream.

The coded data buffer 116, which is coupled to the preparser 112, in theabove described manner, is used to temporarily store the variable lengthencoded data output by the preparser 112 until the syntax parser andvariable length decoding circuit 120 is ready to accept the video datafor processing.

The syntax parser and variable length decoder circuit 120 has an inputcoupled to a data output of the coded data buffer 116 to receivetherefrom the variable length encoded data temporarily stored therein.The syntax parser and variable length decoder ("SP and VLD") circuit 120decodes the received data to a constant length bit stream.

By using the preparser 112 in the above described manner, thecomputation requirements of the SP and VLD circuit 120 are substantiallyreduced as compared to the case where all the received data is syntaxparsed and variable length decoded. The preparser 112 effectively limitsthe number of DCT coefficients which must be variable length decoded forany given macroblock and/or block by discarding coefficients exceedingthe preselected maximum number permitted per macroblock or thepreselected maximum number per block thus reducing the burden on thereal time SP and VLD circuit 120 by putting a clear limit on the amountof data that will have to be processed for each macroblock.

The output of the SP and VLD circuit 120 is coupled to the input of theinverse quantization ("IQ") circuit 122 which, in turn, has an outputcoupled to the input of the IDCT circuit 124.

Reduced complexity IQ and IDCT circuits 122 and 124 are implemented inaccordance with the present invention by intentionally settingpreselected AC DCT coefficients corresponding to each macroblock tozero. The IQ circuit 122 can be designed to preform this function.

The IQ circuit 122 receives the data output by the SP and VLD circuit120 and operates to generate DCT values of zero for preselected DCTcoefficients regardless of the input value of the preselected DCTcoefficients. In this manner, the IQ circuit need not actually processthe preselected DCT coefficients received at its input but only theremaining DCT coefficients.

In one exemplary embodiment, the IQ circuit 122 sets all but the upperleft 4×4 region of DCT coefficients of a set of 8×8 DCT coefficientsrepresenting a HDTV block to zero. FIG. 3A illustrates the resultingeffect on an 8×8 block of DCT coefficients. In the illustration of FIG.3, Xs are used to represent DCT values that are actually processed bythe IQ circuit 122, while as illustrated all the other DCT values areset to zero.

Accordingly, in such an embodiment, the IQ circuit 122 need only includecircuity for performing inverse quantization on 16 of the possible 64DCT coefficients corresponding to each block and circuitry foroutputting zeros for the remaining DCT coefficient values. In thismanner, the inverse quantization computation requirement is reduced by afactor of four. The complexity of the IDCT circuit 124 is reduced in asimilar manner since a preselected number of the DCT values will be setto zero by the IQ circuit 122.

MPEG permits the use of either interlaced pictures or non-interlacedpictures. When encoding video data as interlaced pictures, the DCToperation may have been performed on either a field or frame DCT basis.When interlaced video data is encoded on a frame DCT basis the highfrequency coefficients represent temporal as well as spatialinformation. In such a case it may be desirable to maintain highvertical frequencies to maximize the preservation of temporalinformation.

Accordingly, in one exemplary embodiment in which it is expected thatsome interlaced pictures encoded on a frame DCT basis will be received,the IQ circuit 120 is designed to force to zero all DCT coefficientsexcept the DCT coefficients corresponding to rows 0, 1, 6 and 7 andcolumns 0, 1, 2, 3 of an 8×8 block of DCT coefficients representing atwo dimensional array of DCT coefficients corresponding to a HDTVmacroblock. Such a case is illustrated in FIG. 3B where Xs are used torepresent DCT coefficient values that are processed by the IQ circuit122 and zeros are used to represent the preselected DCT coefficientsthat are set of zero.

It should be noted that it is possible to select DCT coefficients otherthan those illustrated in the embodiments of FIGS. 3A and 3B to be setto zero depending on the desired implementation.

The IDCT circuit 124 receives the data output by the inversequantization circuit 122 and performs an inverse DCT operation on thereceived data. Because, in accordance with the present invention,preselected DCT coefficients were intentionally set to zero, the inverseDCT circuit need not include circuitry for processing thosecoefficients. Thus, as with the inverse quantization circuit 122, theIDCT circuit 124 can be implemented using a relatively simple circuit.

The output of the IDCT circuit 124 is coupled to the input of thedownsampler 126. The downsampler 126 is used to downsample the datacorresponding to each picture prior to storage in the frame buffer 118.As a result of the downsampling operation, the amount of data requiredto represent a video frame is substantially reduced. For example if thedownsampler 126 is implemented to remove half of the digital samplesused to represent a picture, the amount of data that would have to bestored will be reduced by a factor of approximately two substantiallyreducing the amount of memory required to implement the frame buffer118.

Referring now briefly to FIG. 2A there is illustrated another embodimentof the decoder according to the present invention as indicated by thereference numeral 200. Like numbered elements in FIGS. 1 and 2A are thesame or similar to each other and thus will not be described again indetail. In the decoder 200, a small buffer 202, e.g., a buffer capableof storing the data corresponding to several lines of a video frame, incombination with a low pass filter 204, is used to couple the output ofthe IDCT circuit 124 to the input of the downsampler 126. The buffer 202stores video data representing several scan lines of a picture which isthen low pass filtered by the low pass filter 204 prior to downsamplingto reduce the blockiness in the image that might otherwise result fromthe downsampling operation.

Referring again to FIG. 1 it can be seen that the output of thedownsampler 126 is coupled to a first input of the summer 128 and afirst input of the switch 129. A second input of the summer 128 iscoupled to the output of the MCP circuit 130 which supplies motioncompensation information to the summer 128 to be combined with receivedP- and B- video frames. The video data output by the summer 128represents a video frame that has been processed using motioncompensation data to include the relevant video information frompreceding and/or subsequent anchor frames. The output of the downsampler126, on the other hand, represents either fully decoded picture data ordecoded prediction residuals in the cases of intraframe codedmacroblocks and non-interframe coded macroblocks respectively. Theswitch 129 is controlled, as is known in the art, to selectively couplethe output of the downsampler 126 to the input of the frame buffer 118when the received video macroblock was not encoded using motioncompensated prediction, e.g., when the video data output by thedownsampler was intra-frame coded. However, when the video data outputby the downsampler 126 was encoded using motion compensated prediction,e.g., when it is B- frame or P- frame data, the output of the summer 128is coupled by the switch 129 to the input of the frame memory 118.

The received downsampled, decompressed video frames are stored in theframe memory 118. The frame memory 118 has a first output coupled to aninput of the MCP circuit 130 and a second output coupled to an input ofthe post processor circuit 136. In this manner the MCP circuit issupplied with downsampled video frames to be used as anchor frames formotion compensation purposes.

Referring again briefly to FIG. 2A, it can be seen that the decoder 200contains additional circuitry for reducing the amount of data requiredto be stored by the frame buffer 118, i.e., a least significant bit("LSB") truncation circuit 208. The LSB circuit 208 couples the outputof the switch 129 to the input of the frame buffer 118. MPEG assumesthat 8 bits will be used per pel when representing a video frame. Inaccordance with the present invention, in order to reduce frame buffermemory requirements, one or more least significant bits are truncatedfrom each set of bits representing a pel. For example, in oneembodiment, the LSB truncation circuit 208 truncates the datarepresenting a pel from 8 bits to e.g., 6 or 7 bits. When the datacorresponding to each pel is truncated to 6 bits a data savings ofapproximately 25% results.

The amount of truncation performed may be implemented as a function ofthe type of frame to which the data corresponds. Because reduction inthe resolution in anchor frames will impact the quality of frames whichuse these frames for motion compensation purposes, it may be desirableto reduce the resolution of these frames less than frames, such as B-frames, which do not impact the quality of other frames. Accordingly, inone embodiment anchor frame data, e.g., I- and P- video frame data, istruncated to 7 bits per pel while data which is not used for anchorframes, e.g., B- frame data, is truncated to 5 bits per pel.

When LSB truncation is used to reduce frame buffer memory requirements,in order to maintain MPEG compatibility it is necessary to represent thestored data in an 8 bit per pel format. Accordingly, the outputs of theframe buffer 118 are coupled to least significant bit padding circuits206, 207 which pad the least significant bits of the data output by theframe buffer 118 with zeros to insure that the data corresponding toeach pel of a frame is represented by 8 bits. The output of the LSBpadding circuit 206 is coupled to the input of the MCP circuit therebyproviding the MCP circuit with downsampled video frame data. The outputof the LSB padding circuit 207 is coupled to the input of an optionallow pass filter 209 which, in turn, is coupled to the post processingcircuit 136. The optional low pass filter 209 reduces the effect of thequantization noise generated as a result of the truncation and paddingprocess but also results in a slight reduction in the resolution of thevideo frame being supplied to the post processing circuit 136.

The operation of the MCP circuit 130 illustrated in FIGS. 1 and 2A,which is designed to operate using motion vectors generated inaccordance with the MPEG standard, will now be described.

MPEG supports the use of motion compensation and, in particular, P-frames and B- frames each of which rely on another frame as an anchorframe. The motion vectors received as part of the video data streamsupplied to the preparser 112 were calculated at encoding time for usewith full resolution video frames as opposed to downsampled videoframes.

In accordance with the present invention, the downsampled framessupplied by the frame buffer 118 to the MCP circuit 130 are upsampled,e.g., on-the-fly, interpolated and then downsampled prior to generatingpredictions based on the motion vectors. In this manner the motionvectors which were originally generated based on full resolution videoframes are effectively applied to downsampled video frames.

As illustrated in FIG. 1, the MCP circuit 130 comprises an upsampler 131which has an input coupled to the first output of the frame buffer 118,and an output coupled to the input of a half pel interpolation circuit132. The output of the half pel interpolation circuit 132 is coupled tothe input of a motion compensated prediction module 135. The output ofthe motion compensated prediction module 135 is coupled to a downsampler133 which, in turn, has an output coupled to the input of the averagepredictions circuit 134. The output of the average predictions circuit134, which serves as the output of the MCP circuit 130, is coupled tothe second input of the summer 128.

Various known upsampling and down sampling filters may be used for theupsampler 131 and downsampler 133, respectively. However, for bestresults, simulations have shown that it is very important that thedownsampler 133 use a downsampling process which is the exact inverse ofthe upsampling process since even small differences are made noticeablein inter-coded video frames after many generations of predictions.

There are several ways the required upsampling and downsampling can beaccomplished. For example, in one embodiment a bilinear upsamplingfilter with a 3×3 kernel and a center coefficient=1.0 is used as theupsampler 131 and a downsampler which does not perform filtering is usedas the downsampler 133. This selection of filters allows for reasonablygood upsampling with no additional blurring during downsampling. Thisfilter arrangement works especially well when the residual image to beaccumulated during predication has been lowpass filtered prior toaccumulations. Such low pass filtering is, as discussed above, achievedto some extent as a result of the preparser 112 removing higher order ACDCT coefficients from macroblocks which include more than thepreselected maximum number of DCT coefficients per macroblock or byremoving higher order AC DCT coefficients from blocks of a macroblockwhich include more than a preselected maximum number of DCT coefficientsper block.

In another embodiment an upsampling filter with a kernel size of 2×2pixels is used as the upsampler 131 and a downsampling filter is usedwith the same kernel size as the upsampling filter and a transferfunction such that the convolution of the upsampling filter transferfunction and the downsampling filter transfer function produces a 3×3kernel whose center coefficient is 1.0.

While it is preferred that identical downsampling circuits be used forthe downsampler 126, 123 there is no requirement that this in fact bethe case. However, it is important that the downsampler 133 be matchedto the upsampler 131, as discussed above, to provide good results.

MPEG permits the portion of an image corresponding to a macroblock to beencoded either on an interlaced or noninterlaced manner. Macroblockscorresponding to noninterlaced pictures are always coded using a frameDCT format and with macroblocks corresponding to interlaced picturesbeing coded using either a field or frame DCT format.

In accordance with the present invention, downsampling is performedseparately on each individual macroblock. Downsampling of macroblockscorresponding to noninterlaced pictures is performed by the downsampler126 on a frame basis.

However, with regard to macroblocks corresponding to interlaced picturesdownsampling can be performed on either a field or frame basis for eachmacroblock.

As discussed above, the downsampler 126 performs downsampling onnon-interlaced pictures on a frame basis. However, with regard tointerlaced pictures it can be performed on either a field or framebasis. Thus, in the case of macroblocks of interlaced pictures one isconfronted with a decision whether the downsampling, performed by thedownsamplers 126, 133, should be done on a field or on a frame basis.Field based downsampling preserves the greatest degree of temporalresolution whereas frame based downsampling has the potential ofpreserving the greatest degree of spatial resolution. In one embodimentof the present invention pictures are downsampled in a consistent mannereither on a field or frame basis when interlaced pictures are received.Accordingly, in one embodiment both interlaced and noninterlacedpictures are downsampled on a frame basis. While in another embodimentnon-interlaced pictures are downsampled on a frame basis whileinterlaced pictures are downsampled on a field basis.

In another slightly more elaborate embodiment, the downsampler 126performs downsampling on a interlaced pictures on both a field and framebasis with the particular method depending on the individual macroblockbeing processed.

Field based downsampling is most appropriate when there is not much highfrequency content in the macroblock being downsampled and/or there is agreat deal of motion associated with the macroblock. On the other hand,frame based downsampling is most appropriate when there is significanthigh frequency content in the macroblock being processed and/or littlemotion associated with the macroblock. Accordingly, in one embodiment,the downsampler 126 assesses the described characteristics for eachmacroblock of an interlaced picture and dynamically determines the mostappropriate method for downsampling the individual macroblock.

In yet another embodiment which supports downsampling on both a fieldand frame basis but which requires relatively little circuitry fordeciding which down sampling approach to use, the downsampler 126 merelyfollows the choice made by the encoder when it originally encoded themacroblock being downsampled since the criteria used for previouslydeciding whether to encode the macroblock on a field or frame DCT basisis similar to the criteria which should be used to decide whether todownsample on a field or frame basis.

Accordingly, in one embodiment of the present invention the downsampler126 downsamples macroblocks corresponding to an interlaced picture on aframe basis if they were encoded on a frame DCT basis and downsamplesthem on a field basis if they were encoded on a field DCT basis.

While the downsampling of frames prior to storage in the frame buffer114 reduces frame buffer memory requirements, it has the disadvantage ofrequiring the use of the upsampler 131, half pel interpolation circuit132 and downsampler 133 in order to take full advantage of the motionvectors. Such circuitry slightly increases the processing requirementsas compared to those of standard MCP circuits 15 since the anchorpicture data must be upsampled prior to interpolation if half-pelresolution is to be achieved. It should be noted that as an alternativeto upsampling, interpolation, and then downsampling, a less complicatedand thus less costly way to perform motion compensation, with somewhatlower quality results, is to divide the value of each original motionvector by the integer decease in resolution and take the average integerresult of the division operation as the new motion vector value.

While downsampling thus tends to increase MCP processing requirementsslightly it has the advantage of reducing the complexity of theinput/output circuitry to the MCP circuit because the data bandwidthinto and out of the MCP circuit 130 is reduced as a result of thedownsampling by the same factor as the buffer frame memory requirements.

In accordance with one embodiment of the present invention, in order toreduce the processing and bandwidth requirements of the MCP circuit 130,and thus the complexity of the MCP circuit 130, bi-directional anddual-prime prediction is avoided. Thus eliminating the need for theaverage predictions circuit on the ability to process to anchor framessimultaneously. Once a picture has been upsampled by the upsampler 131interpolation is preformed by the half pel interpolation circuit. Themotion compensated prediction module then actually applies the motionvectors to the upsampled picture which is then downsampled by thedownsampler 133. When the picture being processed is a bi-directionallycoded picture, the upsampling interpolation, motion and predictionoperations will be performing on two anchor frames which are thenaveraged together by the average predictions circuit to generate thesingle picture output by the MCP circuit 135.

This is accomplished, in accordance with one embodiment of the presentinvention by using only a single reference or anchor picture forprediction purposes even if the macroblock type flag associated with amacroblock being processed indicates that it is a bi-directional codedmacroblock, i.e., a macroblock that was coded based on the content oftwo different pictures.

In such an embodiment when the bi-directionally coded macroblocks arereceived, the MCP circuit 130 performs motion compensation using onlyone reference despite the fact that the macroblock type flag associatedwith the individual macroblock being processed in the data streamindicates that motion compensation should be performed using tworeferences.

In an exemplary embodiment, the upsampler circuit 131 detects when abi-directionally or dual coded macroblock is being processed, selectswhich of the two possible anchor pictures associated with the macroblockto use for predication purposes and then upsamples only the selectedanchor picture.

In one embodiment, the upsampler circuit 131 selects which of the twopossible anchor pictures to use for predication purposes by selectingthe anchor picture that is temporally closet to the current picture andthus macroblock being processed. If the two anchor pictures aretemporally equidistant, the average predictions circuit 134 selects theearlier of the two possible anchor pictures to thereby select the one ofthe two anchor pictures which will have suffered the least degradationdue to drift.

Since errors do not propagate from B-pictures, because they are not usedas anchors for subsequent pictures, a fairly large degree of distortion,i.e., of the type caused by using only a single anchor picture forprediction purposes, can be tolerated with regard to B-pictures.However, the use of a single anchor picture for prediction purposes withdual-prime coded sequences may result in somewhat less satisfactoryresults because prediction errors may propagate when dual-prime codedsequences are used.

Referring now to FIG. 2B there is illustrated a video decoder circuit300 according to the present invention capable of decoding HDTV signalsat less than full HDTV resolution and SDTV signals at full SDresolution. As illustrated, the decoder circuit 300 is similar to thedecoder circuit 100 but includes additional circuitry, i.e., a HDTV/SDTVmode control circuit 310 for controlling the decoder circuit's mode ofoperation, a set of switches 301, 302, 303 and a multiplexer 341 forbypassing the decoder's preparser 112, downsampler 126, upsampler 131,and downsampler 133 when the HDTV/SDTV mode control circuit 310determines what an SDTV signal is being received. When the HDTV/SDTVmode control circuit 310 determines that a HDTV signal is being receivedit controls switches 301, 302, 303 and multiplexer 341 to connect thecircuit elements in generally the same manner as illustrated in theembodiment of FIG. 1 so that the data reduction features of the decodercircuit 300 of FIG. 2B are operative.

The HDTV/SDTV mode control circuit 310 receives as its input thevariable length encoded bitstream. From analyzing the data in thebitstream, e.g., header information indicating the resolution of thevideo frames being received, it determines if it is receiving HDTV data,i.e., data of a higher resolution than it can decode without performingdata reduction operations on the received data, or SDTV data, i.e. datawhich the decoder 300 can fully decode and store without having toperform any data reduction operations.

The HDTV/SDTV mode control circuit 310 generates a mode control signalwhich is supplied to a position control input of the switch 301 which ispart of the preparser module 312, a position control input of the switch302 which is part of a downsampler module 326 and a position controlinput of the switch 303 and multiplexer 341 which is part of a motionvector interpolation module 333. As discussed above, the output signalof the HDTV/SDTV mode control circuit 310 operates to switch each of themodules 312, 326, and 333 between a first mode of operation, e.g., aSDTV mode of operation, to a second mode of operation, e.g., a HDTV modeof operation. In such a manner, the decoder 300 is controlled by theHDTV/SDTV mode control circuit to perform data reduction to reduce theresolution of the received frames during the second mode, i.e., the HDTVmode of operation, but not during the first mode of operation, i.e. theSDTV mode of operation.

While the frame buffer 118, coded channel buffer 116, IQ circuit 122,IDCT circuit 124, MCP circuit 330 and SP and VLD circuit 120 aregenerally the same in the decoder 300 as the decoder 100, variousfeatures of each of these decoder elements will be discussed as theyrelate to the particular embodiment illustrated in FIG. 2B, e.g., whenthey are implemented as part of a decoder which is capable of decodingboth a television signal of a first resolution and a second televisionwhich is of lower resolution than the first television signal, e.g., HDand SD television signals.

The preparser 112, in the embodiment illustrated in FIG. 2B, is designedto reduce the data rate of a received HDTV signal to approximately thatof a SDTV signal. Thus, the preparser 112 limits the number ofrunlength/amplitude symbols per coded macroblock and the number of bitsper macroblock so that the SP and VLD circuit 120 can function atapproximately the same rate when processing HDTV signals as whenprocessing SDTV signals, e.g., at the MP@ML specified maximum data rate.

This reduces the memory requirements for the coded data buffer 116 whichserves as a channel buffer. Thus, the decoder 300 uses a channel bufferthat meets the memory requirements for a SDTV decoder but which issmaller than that required for a fully MPEG compliant HDTV decoder.

U.S. ATV specifies a channel buffer of 8 Mb, whereas only 1.5 Mb arerequired to implement a MP@ML channel buffer. Because the MP@MLspecification requires 830 Kbits more for a frame buffer than arerequired for decoding reduced resolution video pictures in accordancewith one embodiment of the present invention, this 830 Kbits of theframe buffer which are not used for picture storage during HDTV modeoperation can be used to supplement the memory of the channel bufferwhen operating in HDTV mode. Thus, in accordance with the presentinvention, the coded data buffer is implemented using approximately 2.33Mbits which are also required to implement a full resolution SD decoder.

It is the responsibility of the preparser 112 to insure that the codedchannel buffer 116 does not overflow during HDTV mode operation. This isachieved by, e.g., the preparser 112 discarding the less useful DCTcoefficients in order to prevent the smaller than full size HDTV channelbuffer 116 from overflowing.

While, the SP and VLD unit 120 is designed to operate at the samegeneral data rate as a SDTV data rate, it is also designed toaccommodate the larger range of parameter values allowable for HDTVcoding, as compared to SDTV decoding, e.g., frame rate, picturehorizontal size and picture vertical size. Thus, the SP and VLD circuit120, because of the larger range of parameter values required to supportboth HDTV and SDTV decoding, is slightly more complicated then astandard SDTV syntax parser and variable length decoder circuit in thisregard.

As described above, the output of the syntax parser 120 is coupled tothe input of the IQ circuit 122 which, in turn, is coupled to the IDCTcircuit 124. The IQ and IDCT circuits 122, 124 of the decoder 300 willnow be described.

MPEG MP@ML allows for pixel rates of up to 10.4 million pixels persecond. U.S. ATV allows for pixel rates of up to 62.2 million pixels persecond.

In accordance with the present invention, IQ circuitry which meets MP@MLIQ requirements is used for the IQ circuit 122 of the decoder 300. Insuch an embodiment, the IQ circuit 122 is designed to process all theDCT coefficients of a SDTV macroblock but only, a subset of DCTcoefficients of a HDTV macroblock, e.g., 10 or 11 DCT coefficients ofeach HDTV block including, e.g., a total of 64 DCT coefficients.

That is, the IQ circuit 122 is designed to set a preselected group ofDCT coefficients of a HDTV macroblock to 0 while processing theremaining DCT coefficients of each HDTV macroblock being processed. Inone exemplary embodiment, for HDTV macroblocks coded using field DCTs,the DCT coefficients of an 8×8 block are discarded by the IQ circuit 122as illustrated in FIG. 3C where zeros are used to represent discardedDCT coefficients and X's are used to represent DCT coefficients whichare processed by the IQ circuit 122. For HDTV blocks coded using frameDCTs, the DCT coefficients of an 8×8 block are discarded by the IQcircuit 122 as illustrated in FIG. 3D.

The IDCT circuit 124, like the IQ circuit 122 is designed to processesonly a small percentage of the DCT coefficients associated with a HDTVblock, e.g., the same DCT coefficients processed by the IQ circuit 122.

An IDCT engine capable of performing fast one dimensional 8 point IDCTscan be used to implement the 2-D IDCT operation required by the IDCTcircuit 124. A standard MP@ML IDCT engine can be used to compute aboutthree 8 point IDCTs in the time allowed for IDCT processing of one HDTVblock. Accordingly, in one embodiment of the present invention, onlyDCTs in the first three columns of an HDTV block are processed by theIDCT circuit 124, e.g., the DCT coefficients indicated by Xs in FIGS. 3Cand 3D.

Accordingly, in one embodiment the IDCT circuit 124 includes a MP@MLcompliant IDCT engine for calculating IDCTs of the first three columnsof a HDTV block and all the IDCTs of a SDTV block. Since, in accordancewith the exemplary embodiment of the present invention, the DCTcoefficients of all but the first three columns of a 8×8 HDTV block areset to zero, there is no need to calculate the IDCTs for anycoefficients outside the first three columns.

While the MP@ML IDCT engine is used to calculate the column IDCTs, anIDCT engine for calculating the row IDCTs for a HDTV block is alsoincluded in the IDCT circuit 124. However, in accordance with thepresent invention, this can be a relatively simple IDCT engine since 5of the 8 row coefficients will always be zero by the preselected settingof such coefficients to zero. Furthermore, because only the first two orthree columns contain coefficient values that are to be used in thecalculation of row DCTS only 2 or 3 points need to be calculated foreach row transform. Note that in the embodiment which uses the 8×8 DCTarrangement illustrated in FIGS. 3C and 3D only 4 rows have to betransformed when no additional filtering is performed prior todownsampling by the downsampler 126.

As an alternative method of implementing an IDCT circuit 124 which usesan MP@ML IDCT engine as part of HD decoder IDCT circuit, the lowfrequency rectangle of coefficients from several blocks may be combinedinto a single block as illustrated in FIG. 3E, where "1" is used torepresent low-frequency coefficients from a first block, "2" is used torepresent low-frequency coefficients from a second blocks, "3" is usedto represent low-frequency coefficients from a third block and "4" isused to represent low frequency DCT coefficients from a fourth block.

In accordance with this alternative embodiment, the IDCT circuit 124arranges the DCT coefficients from four different HDTV blocks into asingle 8×8 block of DCT coefficients in the described manner and aconventional 8×8 point IDCT engine is then used to process the compositeblock of DCT coefficients. After processing by the 8×8 point IDCT enginerelatively simple linear operations performed on the result of the IDCToperation are used to generate a good approximation of the IDCTs of thelow frequency coefficients of each one of the four component HDTV blocksused to form the composite coefficient block.

As discussed above, the video picture data output of the IDCT circuit124 is downsampled when the decoder 300 is operating in HDTV mode priorto storing in the frame buffer 118 to thereby further reduce memoryrequirements.

MP@ML allows for pictures as large as 720×576 pels for a total of 414720pels per picture while US ATV allows for pictures as large as 1920×1080pels per picture. Sequences received in the 1920×1080 pel picture formatcan be downsampled by a factor of 3 horizontally and a factor of 2vertically to yield a maximum resolution of 640×540 pels for a total of345,600 pels. In such an embodiment, the same amount of memory requiredto implement a full resolution SD decoder is satisfactory for storing adownsampled HDTV video picture. Thus, the memory provided for MP@MLwould be adequate for the reduced resolution HD decoder as well, with anadditional 830 KBits available for other purposes, e.g., for use as partof the coded data buffer 116 when operating in HDTV mode as describedabove.

By selecting decoder components that are designed in the above manner,it is possible to implement a combined low resolution HDTV decoder andfull resolution SD decoder at a cost that is expected to be about 130%of the cost of a full resolution SDTV decoder. The decoder 300 may beused as part of a television set which is designed to decode and displayboth HDTV and SDTV signals at a relatively low cost as compared to asystem incorporating a full resolution HDTV decoder.

The decoders illustrated in any of FIGS. 1 and 2A-2B may also be used toimplement picture-in-picture capability in a HDTV or SDTV set withoutincurring the cost of having to provide a full resolution decoder forthe decoder used to display the picture shown inside the main picture ofa picture-in-picture display.

For a MPEG compatible digital television set to operate in apicture-in-picture mode, two MPEG decoders must be used, a first decoderto decode the full resolution picture to be displayed on a first portionof the screen and a second decoder to decode the reduced size picturedisplayed on a second smaller portion of the screen.

Referring now to FIG. 4, there is illustrated a picture-in-picture("PIP") decoder 400 in accordance with the present invention. The PIPdecoder 400 receives MPEG compliant demodulated digital videoinformation, e.g., either an SD or HD TV signal, from atuner/demodulator 399 which is supplied to the input of a primary, e.g.,full resolution decoder 401 and to the input of one or more reducedresolution decoders, i.e., the first and second reduced resolutiondecoders 402, 403.

The primary decoder 401 is responsible for decoding the main picture ofa picture-in-picture image while the first and second decoders areresponsible for generating separate images which will be displayed in asmall area of the main picture. A separate reduced resolution decoder402 or 403 is used for each additional image that is to be displayed inaddition to the main picture.

The output of the primary decoder 401 and the reduced resolutiondecoders 402, 403 is coupled to the input of a picture-in-picture videoprocessing circuit which operates to combine the main picture with thereduced resolution pictures output by the reduced resolution decoders402, 403 prior to the resulting combined picture being displayed.

In one embodiment of the present invention the size of the reducedresolution pictures incorporated into the main picture is selected to be1/4×1/4 the size of the normal picture. In such an embodiment, each MPEG8×8 pixel block need only be decoded to a size corresponding to a blockof 2×2 pixels.

The cost of the IDCT circuit 124 used in the reduced resolution decoders402, 403 can be substantially reduced in accordance with the presentinvention by performing the IDCT operations on only 2×2 blocks asopposed to 8×8 blocks. This is achieved by, e.g., retaining andprocessing only the upper left 2×2 block of DCT coefficients of each 8×8DCT coefficient block of a HDTV picture with all the other DCTcoefficients being set to zero. Accordingly, in such an embodiment, theIDCT circuit cost is reduced to approximately the cost of a circuitwhich can perform a 2×2 IDCT as opposed to an 8×8 IDCT.

The IQ circuit 122 of the reduced resolution decoders 402, 403 can besimplified in a similar manner with the IQ circuit 122 operating only ona 2×2 block of DCT coefficients, i.e., 4 coefficient values, as opposedto 64 DCT coefficient values that form an 8×8 DCT coefficient block.

In accordance with the present invention, the reduced resolutiondecoders 402, 403 store the low resolution frames in their reduced size.Thus, by using 2×2 DCT coefficient blocks the size of the frame bufferscan be about 1/16th of the size that would be required if 8×8 DCTcoefficient blocks, i.e., full resolution blocks, were stored.

In addition to reducing frame buffer size the preparser 112 serves toreduce channel buffer requirements and the complexity of the SP and VLDcircuit by, in accordance with the PIP embodiment, discardingamplitude/runlength pairs corresponding to each block that exceed the 4amplitude/runlength pairs that are used by the reduced resolution PIPdecoder 402 or 403.

In a PIP embodiment, the MCP circuit 130 can perform interpolation onthe reference frames for macroblocks whose motion vectors are not evenlydivisible by four to achieve good prediction results. For a lessexpensive and somewhat lower quality implementation, motion vectorscould be rounded to the nearest factor of 4 and applied withoutinterpolation.

While the above PIP decoder embodiment of the present invention has beendescribed in the context of using 1/4×1/4 size pictures for thesecondary pictures inserted into the primary picture, other sizepictures could also be generated at reduced cost, e.g., 1/2×1/2 sizepictures, using the decoder 100 described above. In such a case, thedownsampling would have no be performed in a manner that would producethe desired reduction in size and data rate.

It should also be noted that the arrangement illustrated in FIG. 4 canbe used to provide picture-in-picture capability in a SDTV receiver aswell as a HDTV receiver. Furthermore, the illustrated PIP decoderarrangement is in no way limited to a specific degree of resolution withregard to the primary decoder 401 and only requires that the secondarydecoder(s) 402, 403 be implemented as reduced resolution decoders ascompared to the resolution supported by the primary decoder.

What is claimed is:
 1. A method of decoding a digital video signalincluding a picture of one of a first and a second degree of resolution,the first degree of resolution being a higher degree of resolution thanthe second degree of resolution, the picture including digital videodata, the method comprising the steps of:providing a picture buffer forstoring pictures; receiving the digital video signal including thepicture; monitoring the digital video signal to determine whether thereceived picture is of the first or second degree of resolution; upondetermining that the received picture is of the first degree ofresolution performing the steps of:i. downsampling the picture to reducethe amount of video data included in the picture; and ii. storing thepicture in the picture buffer; upon determining that the receivedpicture is of the second degree of resolution performing the step ofstoring the picture in the picture buffer without downsampling.
 2. Adigital picture-in-picture decoder circuit for decoding a primarypicture to be displayed in a primary picture area and a second pictureto be displayed in a secondary picture area, the secondary picture areabeing smaller than the primary picture area, the decoder circuitcomprising:a full resolution video decoder for receiving and decoding afirst set of digital video data including the primary picture; a firstreduced resolution video decoder for receiving a second set of digitalvideo data including the second picture and for decoding the receivedvideo data to generate a reduced resolution version of the secondpicture; a second reduced resolution video decoder for receiving a thirdset of digital video data including a third picture and for decoding areduced resolution version of the third picture; and a post processorcircuit coupled to the full resolution video decoder and the first andsecond reduced resolution video decoders, the post processor circuitoperating to combine the reduced resolution version of the first fullresolution picture, the reduced resolution version of the second pictureand the reduced resolution version of the third picture to form thesingle picture.
 3. A digital video decoder circuit for decoding aprimary picture represented by a first set of digital video data and asecond picture represented by a second set of digital video data, thesecond set of digital video data including a plurality of macroblocks,each macroblock including a plurality of DCT coefficients, the decodercircuit comprising:a full resolution video decoder for receiving anddecoding the first set of digital video data; a reduced resolution videodecoder for receiving and decoding the second set of digital video datato generate a reduced resolution version of the second picture, thereduced resolution video decoder including:i. preparser means forreceiving the second set of digital video data and for removing fromeach macroblock DCT coefficients that exceed a preselected maximumnumber of DCT coefficients per macroblock; and ii. a channel buffercoupled to the preparser means for temporarily storing the video dataoutput by the preparser means; and a post processor circuit coupled tothe full resolution decoder circuit and the reduced resolution decodercircuit, the post processor circuit operating to combine the primaryfull resolution picture and the reduced resolution version of the secondpicture into a single picture.
 4. The digital video decoder circuit ofclaim 3, wherein the channel buffer is a size which is smaller than thesize required to store the entire second set of digital video data. 5.The digital video decoder circuit of claim 4, wherein the reducedresolution video decoder further comprises:a syntax parser and variablelength decoder circuit coupled to the channel buffer for variable lengthdecoding the second set of digital video data; a downsampler coupled tothe syntax parser and variable length decoder circuit for downsamplingthe variable length decoded video data to thereby reduce the amount ofresolution of the second picture and the amount of video data used torepresent the second picture.
 6. The digital video decoder circuit ofclaim 5, wherein the full resolution video decoder is a HDTV decoder andwherein the reduced resolution video decoder is a reduced resolutionvideo decoder that generates pictures 1/4×1/4 the size of the fullresolution video decoder.
 7. A digital video decoder circuit fordecoding a primary picture represented by a first set of digital videodata and a second picture represented by a second set of digital videodata, the decoder circuit comprising:a full resolution video decoder forreceiving and decoding the first set of digital video data; a firstreduced resolution video decoder for receiving and decoding the secondset of digital video data to generate a reduced resolution version ofthe second picture, the first reduced resolution video decoderincluding:i. a data reduction circuit for eliminating a portion of thedata included in the second set of digital video data; and ii. a channelbuffer coupled to the data reduction circuit for temporarily storing thevideo data output by the data reduction circuit, the channel bufferbeing a size which is smaller than the size required to store the entiresecond set of digital video data; and a post processor circuit coupledto the full resolution decoder circuit and the first reduced resolutiondecoder circuit, the post processor circuit operating to combine theprimary full resolution picture and the reduced resolution version ofthe second picture into a single picture.
 8. The video decoder circuitof claim 7, further comprising:a second reduced resolution video decodercoupled to the postprocessing circuit for receiving and decoding a thirdset of digital video data to generate a reduced resolution version of athird picture; and wherein the post processor circuit operates tocombine the reduced resolution version of the third picture with theprimary full resolution picture and the reduced resolution version ofthe second picture to form the single picture.